This invention relates to a circuit board and in particular is concerned with contact positions on a circuit board, the contact positions comprising a plurality of closely spaced inter-digital contact members.
Circuit boards, as used in terminals and other forms of electrical and electronic apparatus, typically telephone sets as an example, have circuit patterns which include contact positions. Such contact positions may be switch positions for pushbuttons as for a telephone dial, or for other switches. Contact positions may also be provided for contact by contact members on display devices, such as liquid crystal displays, and other devices.
Such contact positions, for various reasons, may be composed of a closely spaced plurality of inter-digital contact members. The close spacing of the contact members gives rise to production difficulties with conventional production procedures. Normally, the circuit pattern is of copper. To avoid oxide problems and for other reasons, at appropriate places, for example at contact positions, the copper is plated with gold, with an underlayer of nickel between the gold and the copper. The use of gold is expensive and it has been proposed to use a carbon ink, applied via a screen printing procedure. This has become possible because the provision of a very low resistance contact is no longer a necessity, with recent advances in technology.
However, when coating with carbon ink over copper, problems arise in obtaining accurate register between layers. With very closely spaced contact members, quite a small misalignment between the copper pattern and the carbon ink can result in electrical shorting between contact members.